Infectioon of susceptible chickens with cell-associated herpesvirus Marek's disease virus (MDV) results in a lymphoproliferative disease, characterized by development of visceral lymphomas. The lymphomas appear to be composed predeominantly of T lymphocytes. Since T cell mediated immunity appears to be operative in immunosurveillance in MD, the T lymphocyte can be either target of viral transformation of sensitized effector cell. The major emphasis of our present and future studies will be to establish the nature of T cells affected in the different modes of interaction with virus and virus-infected cells. The pathogenic effects of MDV on T lymphocytes can be studied in vitro by applying correlates of cellular immune responses. Culture of T lymphocytes from chickens at different stages of MD have been tested for nitrogen responses. Cell separation studies have provided enriched subpopulations of lymphocytes, which are being investigated for various immune reactivities. Normal and MDV-infected chickens are studied comparatively. Immunofluorescent methods are also applied to analysis of subpopulations with respect to tissue specific and oncogenic virus-related cellular antigens. The effects of MDV infection on immune responses of chickens may be followed on the cellular level, as the oncogenic-disease progresses. The immune basis for protective effects of vaccination with non-pathogenic herpesvirus of turkeys (HVT) or attenuated MDV can be analyzed, by applying methods as described above. Enrichment and characterization of the effector-cells elicited by vaccine protection may be possible, using the gradient enrichment methods.